A 67-year-old hospitalized patient with fever and an indwelling catheter has been on prolonged antibiotic therapy. A urine culture shows Gram-negative rods. MAC pale; Blood agar shows brick-red colonies; Oxidase yellow; Motility motile; DNase halo; TSI K/A with no black precipitate. Which organism do you suspect?

Prepare for the CAMLPR Microbiology Test. Utilize flashcards and engaging multiple-choice questions, each equipped with descriptions and explanations. Ready yourself for an excellent performance!

Multiple Choice

A 67-year-old hospitalized patient with fever and an indwelling catheter has been on prolonged antibiotic therapy. A urine culture shows Gram-negative rods. MAC pale; Blood agar shows brick-red colonies; Oxidase yellow; Motility motile; DNase halo; TSI K/A with no black precipitate. Which organism do you suspect?

The key idea is using distinctive colony traits and enzyme activities to identify a hospital-associated Gram-negative rod. Serratia marcescens is famous for producing a brick-red pigment (prodigiosin) on blood agar, which is a strong clue in differentiating it from other Gram-negative rods. This organism is typically oxidase negative, motile, and often DNase positive (giving a DNase halo). On MacConkey agar it usually appears pale because it is a non-lactose fermenter, in contrast to lactose fermenters like Klebsiella or Enterobacter that turn pink. The TSI reaction is usually K/A with no H2S production, indicating glucose fermentation with acid in the butt but not in a blackened precipitate. Putting these findings together—pale MacConkey colonies, brick-red pigment on blood agar, DNase activity, oxidase negative, motility, and a K/A metabolic pattern without H2S—points to Serratia marcescens. In this patient, a nosocomial pathogen known for catheter-associated infections fits the clinical context of prolonged antibiotic exposure and hospitalization.

The key idea is using distinctive colony traits and enzyme activities to identify a hospital-associated Gram-negative rod. Serratia marcescens is famous for producing a brick-red pigment (prodigiosin) on blood agar, which is a strong clue in differentiating it from other Gram-negative rods. This organism is typically oxidase negative, motile, and often DNase positive (giving a DNase halo). On MacConkey agar it usually appears pale because it is a non-lactose fermenter, in contrast to lactose fermenters like Klebsiella or Enterobacter that turn pink. The TSI reaction is usually K/A with no H2S production, indicating glucose fermentation with acid in the butt but not in a blackened precipitate. Putting these findings together—pale MacConkey colonies, brick-red pigment on blood agar, DNase activity, oxidase negative, motility, and a K/A metabolic pattern without H2S—points to Serratia marcescens. In this patient, a nosocomial pathogen known for catheter-associated infections fits the clinical context of prolonged antibiotic exposure and hospitalization.

More practice questions